Techniques of using an SSD (solid state drive) as a cache of a file system have been known. In the SSD, for example, NAND type semiconductor memory devices (NAND memories) or the like are used. In the case where a storage device is configured by using a semiconductor memory such as a NAND memory, transfer process (compaction or garbage collection) is performed.
For example, an SSD using NAND memories is used as a cache of a HDD (hard disk drive), so that it is possible to implement a high speed in comparison with the case where a cache is not used. However, in the NAND memory, since the aforementioned transfer process (compaction or garbage collection) which generally has a large processing load is needed, high speed implementation may be prevented.